The screenplay was originally based on the James Patterson novel CROSS. For marketing purposes, the film title was changed to that of another Patterson novel, "I, Alex Cross". The screenplay, however, is a very loose adaptation, based on CROSS; similarities between book and screenplay are few.
Director Rob Cohen usually hires composer Randy Edelman to write the music scores to his films. However their last collaboration The Mummy: Tomb of the Dragon Emperor (2008) also has some additional music composed by John Debney, who went on to score Alex Cross for Cohen. It is unusual that the director hired the composer of the additional music from his previous film to be the main composer of his next film.
According to the novel, it was not Sullivan but his friend and partner Jimmy "Hats" Galati who killed Alex Cross' wife Maria. Near the end of the novel, John Sampson (Cross' best friend and partner) reveals that years ago, a couple of months after Maria's death, he and detective Rakeem Powell received information that Jimmy was the murderer. Since they did not have concrete evidence to prove Jimmy's guilt, they searched for him several weeks, and finally found and killed him. Sampson could not bring himself to tell that to Cross back then, because he knew that Cross would be unable to deal with that.